1. Field of the Invention
The present invention relates to an image forming apparatus which modulates light with a spatial light-modulation element, and forms an image on a predetermined surface by using an image-forming optical system, where the image is represented by the light modulated by the spatial light-modulation element.
2. Description of the Related Art
The following documents (1) to (3) disclose information related to the present invention.                (1) Japanese Unexamined Patent Publication No. 2001-305663        (2) Japanese Unexamined Patent Publication No. 2001-21830        (3) Akito Ishikawa, “Reduction of Developing Time by Maskless Exposure and Application to Mass Production,” Electronics Packing Technology (Electronics Jissou Gijutsu), Vol. 18, No. 6, (2002) PP. 74-79, Gicho Publishing & Advertising Co., Ltd. (Kabushiki Kaisha Gijutsu Chosa Kai).        
In some conventionally known image forming apparatuses, light is modulated by a spatial light-modulation element, and an image represented by the light modulated by the spatial light-modulation element is formed on a predetermined surface by using an image-forming optical system. This type of image forming apparatus is widely used to constitute apparatuses which expose an image by projecting an image formed with modulated light on a photosensitive material, apparatuses which display an image by projecting an image formed with modulated light on a screen, and the like. In addition, the above type of image forming apparatus basically comprises a spatial light-modulation element and an image-forming optical system. In the spatial light-modulation element, a great number of pixel portions are two-dimensionally arranged, and each of the pixel portions modulates light applied to the pixel portion according to a control signal. The image-forming optical system forms an image represented by the light modulated by the spatial light-modulation element.
Document (3) listed above and Japanese Patent Application No. 2002-149886 each disclose an exposure apparatus as an example of an image forming apparatus having the above-mentioned basic construction.
Incidentally, in the above image forming apparatuses, the images projected on the photosensitive material or screen are often required to be magnified. In such cases, a magnified-image-forming optical system is used as the above image-forming optical system. In this case, when the light modulated by the spatial light-modulation element simply passes through the image-forming optical system, the light beams from the respective pixel portions of the spatial light-modulation element are widened, and the pixel size in the projected image becomes great. That is, the sharpness of the image decreases.
In consideration of the above circumstances, for example, Japanese Patent Application No. 2002-149886 discloses another construction. In this construction, a first image-forming optical system is placed so that light modulated by a spatial light-modulation element passes through the first image-forming optical system, and a microlens array is placed in an image-forming plane of the first image-forming optical system. In the microlens array, a plurality of microlenses are arranged in correspondence with pixel portions of the spatial light-modulation element, respectively. In addition, a second image-forming optical system is placed so that the light which has passed through the microlens array passes through the second image-forming optical system, and the second image-forming optical system forms an image represented by the modulated light on a photosensitive material or a screen. The first and second image-forming optical systems are arranged so as to magnify the image projected on the photosensitive material or the screen. In this construction, although the size of the image projected on the photosensitive material or the screen is increased, a plurality of portions of light (light beams) from the pixel portions of the spatial light-modulation element are individually condensed by the respective microlenses constituting the microlens array, and therefore the pixel size (spot size) in the projected image is reduced, i.e., is maintained small. Thus, the sharpness of the image can be maintained high.
Although document (1) discloses the construction in which the spatial light-modulation element and the microlens array are combined, the magnification of the image is not considered in document (1).
In addition, in order to improve the extinction ratio in the construction in which the first and second image-forming optical systems and the microlens array are combined, it has been considered to arrange an aperture array so that apertures of the aperture array are located at positions at which a plurality of portions of light from the microlenses converge, respectively. In this case, it is possible to prevent entrance of stray light into optical paths of the plurality of portions of light modulated by the pixel portions of the spatial light-modulation element, and improve the extinction ratio. Further, document (2) discloses an example of a construction in which a microlens array and an aperture array are combined.
When an attempt is made to achieve a great magnification ratio by using first and second image-forming optical systems as mentioned above in a construction in which the first and second image-forming optical systems and a microlens array are combined as above, some problems can occur depending on magnification powers set in the first and second image-forming optical systems.
That is, when the magnification power in the second image-forming optical system is set to one, and the first image-forming optical system is arranged to have a great magnification power, the lens performance (distortion characteristics) and the extinction ratio deteriorate. This is because when the distortion characteristics of the first image-forming optical system deteriorate, a portion of light modulated by a pixel portion of the spatial light-modulation element may be partially mixed into a microlens corresponding to an adjacent pixel portion of the spatial light-modulation element, and the extinction ratio significantly decreases.
On the other hand, when the magnification power in the first image-forming optical system is set to one, and the second image-forming optical system is arranged to have a great magnification power, it is necessary to limit the beam diameters at the light-condensing positions in the microlenses to a very small amount in consideration of the great magnification power of the second image-forming optical system. Therefore, the entire image-forming optical system is sensitive to errors, and designing such an optical system is not easy.
Further, since, in the construction disclosed in document (2), the focal lengths of the microlenses are about 5 mm, and the numerical apertures (NAs) of the microlenses are small, precise setting of the position of the aperture array in the direction of an optical axis are not considered. However, when a microlens array constituted by microlenses each having a great numerical aperture (NA) and a focal length of about 250 to 300 micrometers is used, it is necessary to precisely set the position of the aperture array in the direction of the optical axis so that apertures of the aperture array are located at positions at which a plurality of portions of light from the microlenses converge, respectively. If the error in the position of the aperture array is great, problems occur. For example, the extinction ratio deteriorates due to entrance of stray light, or increase in the amount of light eclipsed by the circumferences of the apertures reduces the light utilization efficiency.